It is often desirable to determine a fluid level in a closed reservoir or container. For instance, it is important to determine the fluid level in an above ground tank, often over an extended period of time. Above ground tanks typically have a capped opening at the top of the tank where a conventional dip stick may be inserted for measuring the fluid level. As the capped openings are often positioned some distance off the ground, typically one or several stories above ground, it may be inconvenient and potentially dangerous for an operator to climb up to the capped opening to perform the measurement. Moreover, measuring the fluid level with a conventional dip stick requires removal of the cap, whereby vapors can escape from the interior of the tank. It will be appreciated that this can be a significant disadvantage for tanks which contain fluids of environmental concern, particularly when periodic or repeated measurements are desired. Thus, there is a continuing need for improved apparatus and methods for measuring a fluid level.